GaAs focal plane array readout

ABSTRACT

A capacitive feedback transimpedance amplifier 10 comprises a differential amplifier constructed of GaAs transistor Q 1  and Q 2  and has an input coupled to the output of an infrared detector 12 and an output expressive of the detector signal integrated over a predetermined interval of time. The amplifier has a transistor Q 0  coupled to the input for coupling the input periodically to a predetermined voltage potential and a transistor Q 9  coupled to the output of the amplifier for simultaneously periodically coupling the output to a predetermined voltage reference, thereby initializing the amplifier at the beginning of an integration period. A load is coupled between an amplifier output terminal a source of amplifier power. The load includes a resistor having a first terminal coupled to the amplifier output terminal and a second terminal coupled to a first GaAs load transistor. The first load transistor is serially coupled to a second load transistor. A gate terminal of each of the first and the second load transistors is coupled to the amplifier output terminal.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This patent application is related to copending U.S. Patent Application S.N. 07/151,845, filed Feb. 3, 1988, R. N. Sato, entitled "GaAs Capacitive Feedback Transimpedance Amplifier". This copending patent application is assigned to the assignee of the present patent application.

FIELD OF THE INVENTION

This invention relates to a Gallium Arsenide (GaAs) focal plane array (FPA) readout electronics circuit including a Capacitive feedback Transimpedance Amplifier, also referred to as a CTIA, operated at cryogenic temperatures. In particular, this invention relates to a GaAs CTIA having a novel load structure and other features that result in an improved immunity to process-related variables.

BACKGROUND OF THE INVENTION

FPAs are typically comprised of a two-dimensional array of Infrared (IR) radiation detectors. The individual IR detector elements may be organized in a regular row and column, mosaic-type fashion Such an array of IR detector elements can be comprised of, for example, HgCdTe, InSb, Multiquantum Well Superlattice (MQW SL) material or doped silicon semiconductor material. The IR detector induced signal from each of the IR detector elements is typically coupled to an electronic interface circuit such as a CTIA, a source follower direct readout or a charge coupled device, where each of the signals are integrated over an interval of time and subsequently read out by a suitable multiplexing circuit. Electronic circuits with HgCdTe detector arrays for processing IR induced signals require a significant degree of material compatibility to achieve a long lifetime and reliable operation. Also, interface electronic circuits require low device and circuit noise characteristics for obtaining a satisfactory signal-to-noise ratio. For some applications the circuits must tolerate nuclear radiation in high nuclear radiation environments and also consume low power to achieve both weight and size reduction.

Typically, readout chips are coupled to, or "bumped", with detector arrays at room temperature using indium bump technology. When the chips are rapidly cooled from room temperature to a cryogenic operating temperature stress can build up between the readout circuit and the detector array through the bumping. This stress is most severe if the coefficients of thermal expansion between the materials are different and/or if the chips are large in area. Silicon, the conventional semiconductor material employed for readout circuits, has a poor thermal expansion match with detector materials such as HgCdTe, InSb and GaAs based MQWL SL IR detectors However, other materials such as GaAs are known to have a coefficient of thermal expansion that is closer to that of these detector materials than is the coefficient of expansion of silicon.

It is therefore one object of the invention to provide a GaAs FPA signal processor that exhibits a low power consumption and an improved immunity to nuclear radiation (radiation hardness).

It is a further object of the invention to provide a GaAs FPA signal processor that exhibits low noise when employed in a FPA signal processor

It is still one further object of the invention to provide a GaAs FPA signal processor array that has a coefficient of thermal expansion which is closely matched to that of the material of an associated IR detecting array.

Another object of the invention is to provide a GaAs FPA signal processor array that includes a differential pair having an improved load transistor circuit that is substantially insensitive to processing-related variables.

SUMMARY OF THE INVENTION

In accordance with the invention a transimpedance amplifier includes a differential amplifier constructed of GaAs transistors and has an input for coupling to an output of an infrared detector. The amplifier has an output terminal expressive of the detector signal integrated over an interval of time. A load is coupled between the amplifier output terminal and an amplifier power supply. The load includes an implant resistor having a first terminal coupled to the amplifier output terminal and a second terminal coupled to a first GaAs load transistor. The first load transistor is serially coupled to a second GaAs load transistor. A gate terminal of each of the first and the second load transistors is coupled to the amplifier output terminal. This load circuit is shown to beneficially reduce an effect of processing and transistor parameter variations upon the transimpedance amplifier circuit performance.

That is, and in accordance with an aspect of the invention, two transistors Q_(L2) and Q_(L3) and one implant resistor R₄ are provided as a load for an amplifier load transistor Q_(L1). An amount of current flowing through Q_(L2) is a function of the magnitude of the resistance of the resistor R₄, R₄ being connected from the gate terminal to the source terminal of Q_(L2). The amount of current flowing is also a function of the threshold voltage, transconductance, and channel conductance of Q_(L2). Any variation of this threshold voltage transconductance or channel conductance is compensated for by the resistor R₄. The gate of Q_(L3) is connected in common with the gate of Q_(L2) to one side of the resistor R₄ such that the gate terminal senses the amplifier output voltage variation. Q_(L3) functions as a source follower coupled to the drain of Q_(L2) such that the voltage between the drain and the source of Q_(L2) is forced to maintain a substantially constant voltage level. Thus, both transconductance and channel conductance of Q_(L2) remain substantially constant.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will become more apparent herein in the detailed description of the invention taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram showing the GaAs readout of the present invention;

FIG. 2 is a schematic diagram that shows in detail the circuitry of the block diagram of FIG. 1; and

FIG. 3 is a timing diagram that illustrates the operation of the circuitry of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 there is illustrated in block diagram form a readout unit cell 10. The unit cell 10 is comprised of a CTIA 14 coupled to a photovoltaic p-n or n-p or a photoconductive infrared (IR) detector 12. IR detector 12 is biased to a desired operating point by V_(det) and is coupled to an input of the CTIA 14. The CTIA 14 is controlled by a plurality of clocks, such as integration command clocks φ1A and φ1B. CTIA 14 is also biased by signals V_(bias), V_(Ibias) and V_(rst). The CTIA 14 is powered by a single power supply voltage (Vdd). The IR induced signal which is integrated by CTIA 14 is provided to a Sample and Hold (S/H) and Multiplexer (MUX) circuit 16. S/H and MUX 16 is controlled by clocks (φ2 and φ3). The multiplexed output of the S/H 16 is a unit cell sampled analog signal (OUTPUT).

Referring now to FIG. 2 there is shown a schematic diagram of a presently preferred embodiment of a FPA signal processor unit cell 10. The unit cell 10 is comprised of 13 GaAs metal-semiconductor field effect transistors (MESFETs), five implant resistors and three capacitors. Switching transistors Q₀, Q₉ and Q₁₁ are enhancement mode devices. A photovoltaic or photoconductive semiconductor radiation detector, which may be an IR detector 12, senses the variation in photon flux within a specified spectral wavelength range and generates a CTIA input signal voltage that is coupled to the gate of Q₁. During operation the GaAs CTIA circuit beneficially provides a constant bias voltage feedback signal to the IR detector 12 through a feedback capacitance C_(f), the feedback signal having a magnitude linearly related to a magnitude of a CTIA 14 output signal appearing at point E₁.

CTIA 14 includes a differential amplifier pair Q₁ and Q₂. In accordance with an aspect of the invention Q₁ has a load represented by transistors Q_(L1), Q_(L2), Q_(L3) and a resistive impedance represented by an implant resistor R₄. These load devices are serially coupled to a drain terminal of Q₁. A source terminal of each of the differential amplifier pair Q₁ and Q₂ is coupled to the drain of a transistor Q₃. Q₃ functioning as a current source. A bias voltage signal V_(Ibias) has a magnitude the value of which is selected to provide a desired current flow through Q₃ to both Q₁ and Q₂. Another source of voltage bias, V_(bias), is provided to the gate of load transistor Q_(L1) and to the drain of Q₂. The voltage magnitude of V_(bias) establishes the drain voltages of both Q₁ and Q₂ at a desired level. In that both Q₁ and Q₂ have substantially identical drain, gate and source voltage levels the devices can be considered to be operated under nearly identical bias conditions, thus ensuring true differential amplifier performance.

In order to realize high-yield production, nonuniformity in chip fabrication must be taken into consideration. Chip fabrication typically causes variations in transistor threshold voltage, transconductance, and channel conductance. In accordance with an aspect of the invention fabrication process variations are compensated for by several different mechanisms. First, V_(bias) compensates for the threshold voltage variation of the differential pair transistors Q₁ and Q₂. Second, a switching transistor Q₉ applies a bias potential V_(rst) to force the CTIA output voltage to a desired initial condition regardless of Q_(L1) transconductance, channel conductance and threshold voltage variations. Third, the CTIA 14 is designed such that the V_(Ibias) voltage provides for a specified low power consumption of the CTIA 14. Finally, the operation of load transistor Q_(L1) is substantially process independent. That is, Q_(L1) is required only to exhibit a relatively low channel conductance because it functions as a source follower. An amount of current flowing through Q_(L2) is a function of the resistance of the implant resistor R₄ connected between the gate terminal and the source terminal of Q_(L2) and also of the threshold voltage of Q_(L2). Any variation of this threshold voltage, however, is compensated for by the resistor R₄.

The voltage at the gate of Q_(L2) varies according to the amount of the photon-induced signal at the CTIA output due to the fact that Q₄ functions as a source follower and the gate of Q₄ is connected to the node that includes the drain of Q_(L1). As a result of this voltage variation the source voltage of Q_(L2) varies significantly, but the drain-to-source voltage of Q_(L2) remains substantially constant. If such a drain-to-source voltage variation were to occur the value of both the transconductance and the channel conductance of Q_(L2) would vary, resulting in a nonuniformity in load resistivity. In order to maintain the Q_(L2) drain-to-source voltage at a substantially constant level transistor Q_(L3) is included as a part of the load circuit The gate of Q_(L3) is connected in common with the gate of Q_(L2) to one side of resistor R₄ such that the gate terminal senses the CTIA output voltage variation. Q_(L3) functions as a source follower coupled to the drain of Q_(L2) such that the voltage between the drain and the source of Q_(L2) is forced to maintain a substantially constant voltage level. Both the transconductance and channel conductance of Q_(L3) are not expected to be constant, but the effect of their variation does not have significant effect since the load represented by Q_(L3) as a source follower is a relatively high resistance. C_(BY) functions as a high frequency bypass capacitor for the differential pair output node.

Switching transistors Q₀ and Q₉, through Q₄, turn on to discharge the feedback capacitor, C_(f), at the beginning of each integration period. Transistor Q₀ provides a specified bias voltage across the infrared detector at each integration period. The switching of Q₉ is controlled by command clock φ1B coupled to the gate thereof, the drain of Q₉ being biased by V_(rst). The voltage level of V_(rst) is determined by the polarity of the associated IR detector 12. That is, the signal input to Q₁ can be either positive or negative depending on the polarity of V_(det) and whether detector 12 is a p-on-n device or an n-on-p type of device. Transistor Q₀ acts as a switch, similar to Q₉, and is turned on periodically by clock φ1A according to the required integration period When both Q₉ and Q₀ are turned on, the feedback capacitance C_(f) is discharged to an initial condition through Q₀ to ground and through the source follower Q₄ to V_(rst) through Q₉. Simultaneously with the discharging of C_(f), Q₀ forces the gate of Q₁ to a desired voltage potential, such as ground, to establish a specified potential across the IR detector 12.

The IR detector induced signal is amplified by differential pair Q₁ and Q₂ in conjunction with Q_(L1), Q_(L2) and Q_(L3) after which the amplified signal is provided to buffer transistor Q₄, which is operated in a source follower configuration. Buffer transistor Q₄ has coupled to the source terminal thereof the aforementioned feedback capacitor C_(f). In general, buffer transistor Q₄ provides a low-impedance source to the feedback path through C_(f) and also to the S/H circuit including Q₈ and signal storage capacitor C_(smp).

Transistor Q₇ functions as a load for the transistor Q₄ in conjunction with a resistor R₅, the resistor R₅ serving to control current flow through Q₇ and hence specify power consumption. V_(bias) is coupled to the gate of Q₇ for controlling the conduction thereof.

The aforementioned S/H circuit is comprised of a GaAs MESFET switching transistor Q₈ and the signal storage capacitor C_(smp). The source-to-drain threshold voltage of Q₈ is sufficiently high to prevent the gate from becoming forward biased.

Transistor Q₁₀ functions as an amplifier and delivers a sampled analog signal (OUTPUT) to a chip driver (not shown). Q₁₀ forms part of the MUX circuit that includes a source resistor R1 and a load resistor R2. R1 and R2 typically have the same resistance value in order to achieve a unity gain feedback amplifier at the MUX output. The particular values of these resistors depends upon the desired multiplexing rate and power consumption. Q₁₀ senses the stored charge across the sampling capacitor C_(smp). Q₁₀ is activated by the φ3 clock which is applied to the gate of Q₁₁. When φ3 is applied, Q₁₁ is turned on thereby providing current through R1 to the unity gain wideband amplifier Q₁₀. The signal OUTPUT has a voltage magnitude substantially equal to the voltage across C_(smp), or the sampled analog signal.

Referring now to FIG. 3 a timing diagram of a single integration period illustrates, in accordance with a method of the invention, the operation of the circuitry shown in FIG. 3. The signal appearing at the CTIA 14 output, designated E₁ in FIGS. 1 and 2, is the integrated IR induced signal. The signal is integrated over an integration period and has an integration slope that is determined by a combination of the detected IR photons, the detector characteristics and the value of the feedback capacitor, C_(f). The clocks φ1A and φ1B occur at substantially the same time during the integration period, the φ1B signal however, having a slightly longer pulse duration A positive going pulse, φ1A, turns on transistor Q₀ thereby effectively shorting the input of CTIA 14 to circuit ground. This has the effect of discharging the feedback capacitor C_(f), thereby resetting the feedback capacitor. φ1B turns on transistor Q₉, thereby establishing the desired potential, V_(rst), at the circuit node that comprises the drain of Q_(L1), the gates of load transist Q_(L2) and Q_(L3) and the gate of buffer transistor Q₄. The output of the CTIA 14 is thereby reset to a desired operating voltage potential at the beginning of each integration period by selecting the V_(rst) voltage level.

The clock φ1B has a longer duration than that of φ1B such that the operating point for the CTIA 14 may be established subsequent to the resetting the CTIA output voltage Shortly after the clock signal φ1B is turned off, thereby cutting off transistor Q₉ conduction, the φ2 clock is turned on which causes the voltage across capacitance, C_(smp), to be the same as that of the CTIA buffer output. The φ2 clock is applied at the beginning of the integration period. The clock φ3 is applied to read the initial value of the CTIA output signal which is stored by the S/H circuit at the beginning of the integration period. The S/H circuit samples the CTIA output and stores the charge across C_(smp). The φ2 clock is again applied to the gate of Q₈ to sample the final value of the CTIA output before the end of the integration period. Clock φ3 is once more applied to sense the voltage across C_(smp). Thus, the MUX circuit output provides two sampled analog pulses per integration period, the magnitude of the pulses corresponding to the beginning and end values of the CTIA output. Determining the difference between the two pulses provides for reducing the effect of amplifier noise originating in Q₁ and Q₂ (1/f noise) and also switching transient noise (kTC noise). Devices Q₁, Q₂, Q_(L1), Q_(L2), and Q_(L3) are designed to achieve a high open loop amplifier gain. The high gain amplifier in conjunction with feedback capacitor C_(f) causes the capacitive feedback to dominate, thereby minimizing the effect of any intrinsic capacitance associated with the IR detector 12 and/or any parasitic capacitance associated with the coupling between the detector 12 and the CTIA 14 input. It can be seen that the magnitude of the CTIA output voltage depends only upon the capacitance of the feedback capacitor Cf and the IR detector 12 induced current. As a result, the uniformity of the readout output voltage of the array is excellent.

It should be realized that the FPA readout of the invention, that is an FPA readout using GaAs semiconductor material, has a number of advantages over a FPA readout comprised of Si. As has been stated, the FPA signal processor of the invention has a similar thermal expansion coefficient as that of an associated HgCdTe, InSb, or GaAs based MQW SL IR detector array. Thus, thermal cycling between room and cryogenic temperatures does not induce mechanical stresses. Furthermore, the higher electron mobility of GaAs results in transistors having a transconductance value equivalent to that of silicon transistors but with a substantial reduction in operating power.

The novel load circuitry disclosed herein provides for advantageously rendering the differential amplifier pair substantially immune to process-related and other variations in circuit parameters. The circuitry also sets the operating point of the CTIA amplifier to a desired condition at the beginning of each integration period while also providing for the biasing of the associated IR detector at an optimum operating point.

The invention can be constructed as shown with GaAs MESFETs or with other types of GaAs transistors such as High Electron Mobility Transistors (HEMTs), Selectively Doped Heterostructure Transistors (SDHTs), Modulation Doped Field Effect Transistors (MODFETs), single quantum or double quantum well transistors, or superlattice MESFETs.

Thus, while the invention has been particularly shown and described with respect to a presently preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention. 

What is claimed is:
 1. A capacitive feedback transimpedance amplifier comprising:means, having an input terminal for coupling to an output signal of an infrared detector, for amplifying the detector output signal to produce at an amplifier output terminal an output signal; means, coupled between said output terminal and said input terminal, for capacitively feeding back to said input terminal a feedback signal having a magnitude related to a magnitude of said output signal; and load means coupled between said amplifier means output terminal and a source of amplifier means power, said load means comprising a resistor having a first terminal coupled to said output terminal and a second terminal coupled to a first load transistor, said first load transistor further being serially coupled to a second load transistor, a control terminal of each of said first and said second load transistors being coupled to said output terminal.
 2. A capacitive feedback transimpedance amplifier as defined in claim 1 wherein said means for feeding back comprises:a buffer amplifier having a buffer amplifier input terminal coupled to said output terminal, said buffer amplifier further having a buffer amplifier output terminal; and a capacitor having a first terminal coupled to said buffer amplifier output terminal and a second terminal coupled to said amplifier input terminal for coupling said feedback signal from said buffer amplifier output terminal to said input terminal.
 3. A capacitive feedback transimpedance amplifier as defined in claim 1 wherein said amplifying means comprises:a differential amplifier comprising a first and a second GaAs transistor connected in parallel one to another, each of said transistors having a source terminal coupled to an output of a current source means, said first transistor having a gate terminal coupled to said detector output signal, said second transistor having a gate terminal coupled to a circuit ground, said first transistor having a drain terminal coupled to a source terminal of a GaAs load transistor, said load transistor having a drain terminal coupled to said amplifier means output terminal, said load transistor further having a gate terminal coupled to a first source of bias potential and also to a drain terminal of said second transistor.
 4. A capacitive feedback transimpedance amplifier as defined in claim 3 wherein said current source means comprises:a third GaAs transistor having a source terminal coupled through a resistor to circuit ground, a drain terminal coupled to said source terminals of said first and said second amplifier transistors and a gate terminal coupled to a source of reference current having a predetermined magnitude for setting the current flow through said first and second transistors at a desired operating power.
 5. A capacitive feedback transimpedance amplifier as defined in claim 1 and further comprising:means for storing a charge signal having a magnitude linearly related to a magnitude of said radiation detector output signal; means, coupled between said buffer amplifier output terminal and said storing means, for periodically impressing upon said storing means said charge signal; and means, coupled to said storing means, for periodically outputting a voltage signal having a magnitude linearly related to the magnitude of said infrared detector output signal.
 6. A capacitive feedback transimpedance amplifier as defined in claim 5 wherein said impressing means comprises a GaAs MESFET transistor having a drain and a source terminal coupled between said buffer amplifier output terminal and said storing means, said GaAs MESFET transistor further having a gate terminal coupled to a clock signal that periodically causes said GaAs MESFET transistor to conduct thereby impressing upon said storing means said buffer amplifier output signal.
 7. A capacitive feedback transimpedance amplifier as defined in claim 1 and further comprising:means, coupled to said input terminal, for periodically coupling said input terminal to a first predetermined voltage potential; and means, coupled to said output terminal, for periodically coupling said output terminal to a second predetermined voltage potential.
 8. An infrared detector readout integrated circuit for use at cryogenic temperatures having a plurality of transistor devices comprised of GaAs, said circuit comprising:a differential amplifier including a first and a second GaAs amplifier transistor connected in parallel one to another, each having a source terminal coupled to an output of a GaAs current source transistor, said first amplifier transistor having a gate terminal for coupling to an infrared detector output signal, said second amplifier transistor having a gate terminal coupled to a circuit ground, said first amplifier transistor having a drain terminal coupled to a source terminal of a first GaAs load transistor, said first load transistor having a drain terminal coupled to an amplifier output terminal, said first load transistor further having a gate terminal coupled to a first source of bias potential and also to a drain terminal of said second amplifier transistor; and load means coupled between said amplifier output terminal and a source of amplifier power, said load means comprising a resistor having a first terminal coupled to said amplifier output terminal and a second terminal coupled to a second GaAs load transistor, said second load transistor further being serially coupled to a third GaAs load transistor, a control terminal of each of said second and said third load transistors being coupled to said amplifier output terminal.
 9. A circuit as set forth in claim 8 and further comprising:a first GaAs switching transistor coupled between said gate terminal of said first amplifier transistor and a first voltage potential, said first switching transistor including a gate terminal coupled to a first clock signal for periodically resetting a first amplifier transistor gate potential to the first voltage potential; and a second GaAs switching transistor coupled between said amplifier output terminal and a second voltage potential, said second switching transistor further having a gate terminal coupled to a second clock signal for periodically resetting an amplifier output potential to the second voltage potential.
 10. A circuit as set forth in claim 9 and further comprising:a GaAs buffer transistor having an input terminal coupled to said amplifier output terminal, said buffer transistor further having an output terminal; a first capacitor having a first terminal coupled to said buffer transistor output terminal, said first capacitor having a second terminal coupled to said gate terminal of said first amplifier transistor for feeding back a signal from said buffer transistor output terminal to said gate terminal of said first amplifier transistor; a third GaAs switching transistor having a drain and a source terminal coupled between said buffer amplifier output terminal and a second capacitor, said third switching transistor further having a gate terminal coupled to a third clock signal which periodically causes said third switching transistor to conduct thereby impressing upon said second capacitor said buffer amplifier output signal; and an output GaAs transistor having a gate terminal coupled to said second capacitor and a source terminal coupled to a fourth clock signal for periodically outputting a signal having a magnitude which is a function of a magnitude of the infrared detector output signal.
 11. A circuit as set forth in claim 10 wherein said current source transistor has a source terminal coupled to circuit ground, a drain terminal coupled to said source terminals of said first and said second amplifier transistors, and a gate terminal coupled to a source of bias current.
 12. A circuit as set forth in claim 11 and further comprising a second GaAs current source transistor having a source terminal coupled to circuit ground and a drain terminal coupled to said output terminal of said buffer transistor, said second current source transistor having a gate terminal coupled to said source of bias current
 13. A circuit as set forth in claim 12 wherein said source terminals of each of said first and said second current source transistors are coupled to circuit ground through an associated current limiting resistance. 